The presence of cancer cells in sentinel lymph nodes is an indication that a cancer has metastasized. Pathological evaluation of these lymph nodes is presently used for staging of melanoma and breast cancers. The sentinel lymph node must first be detected and distinguished from other lymph nodes. This is accomplished by use of radiotracers and blue dye. Radioactive tracer (such as Tc-99m sulfur colloid) is injected around the tumor. This tracer then migrates to the lymph nodes. Utilizing a gamma camera, the sentinel lymph node which now contains the radioactive tracer is located. The patient is then moved to the operating room, where the surgeon injects blue dye around the tumor, and makes an incision in the region marked as being over the area of the sentinel lymph node. The surgeon locates the sentinel node using a gamma ray detecting probe. The sentinel node is then further identified and distinguished from other nodes by visual inspection as to which of the lymph node contains the blue dye that was injected around the tumor. The whole lymph node is then resected and sent to the pathology lab for analysis. In the laboratory, several slices of the removed node are prepared and examined under a microscope to determine if any cancer cells are in that sentinel lymph node.
One of the negative aspects of this procedure is that the whole lymph node must be removed for pathological examination, which requires a surgical incision. The pathologist must then prepare numerous slices of the lymph node and examine each slice, in order to determine if the sentinel node is completely free of cancer cells (tumor-negative.) Even one cancer cell would render the sentinel node tumor-positive and would put the patient into stage 3, requiring the surgeon to perform a complete lymph node dissection to locate that cancer cell.
Another reason that the sentinel lymph node biopsy is done in an open surgery operation (and not percutaneous biopsy) is that the surgeon has to avoid inadvertently cutting blood vessels or nerves during the procedure. Kelcz, U.S. Pat. No. 6,512,943, teaches the use of ultrasound imaging in conjunction with a gamma detection probe for percutaneous localization of the sentinel lymph node. Obtaining ultrasound images helps the surgeon from inadvertently cutting blood vessels or nerves, and helps the surgeon to insert a biopsy needle percutaneously to reach the sentinel lymph node or other lymph nodes. While Kelcz suggests that a percutaneous biopsy is adequate, practitioners have found that such a procedure is not effective in obtaining a reliable analysis of the sentinel node for cancer cells. Since the whole lymph node is still needed for the pathologist to assess if the lymph node contains any cancer cell, and it is nearly impossible to take out the lymph node intact or remove it in its entirety through the biopsy needle, this technique is of very limited value as this procedure can only determine that lymph node is positive if the partial sample happens to contain any cancerous cell. The procedure also lacks negative predictive value.
Examples of the several different designs for intraoperative radiation detection probes which might be used in the new procedures described herein include                Scintillator-PMT systems, that use vacuum tube PMTs and scintillation crystals such as NaI(Tl),        Scintillator-PIN diode systems that use PIN diodes as light detectors and then couple them to a scintillator with emissions around ˜500 nm wavelength (such as CsI). The PIN diode has a gain of one (1) and therefore needs very low noise and high gain amplifiers,        Cd—Te semiconductor detectors, that convert the energy from radiation directly to an electronic pulse        Zn—Cd—Te semiconductor detectors that convert the energy from radiation directly to an electronic pulse.        
A more recent development is a solid state or silicone photomultiplier (SSPM, or SiPM) developed by a team from the Moscow Engineering and Physics Institute (B Dolgoshein Int. Conf. on New Developments in Photodetection (Beaune, France) June 2002) together with Pulsar Enterprise in Moscow which promises a wide range of applications. The device is basically a large number of microphoton counters (1000/mm2) which are located on a common silicon substrate and have a common output load. Each photon counter is a small (20-30 μm square) pixel with a depletion region of about 2 μm of less than 0.1 photoelectron. They are decoupled by polysilicon resistors and operate in a limited Geiger mode with a gain of approximately one million. This means that the SiPM is sensitive to a single photoelectron, with a very low noise level SiPM pixel operates digitally as a binary device, as a whole the SiPM is an analogue detector that can measure light intensity within a dynamic range of about 1000/mm2 and has excellent photon capability.
The pulse height spectrum of such a device is shown in FIG. 23. The photon detection efficiency of the SiPM is at about the same level as photomultiplier tubes (PMTs) in the blue region (20%), and is higher in the yellow-green region. The device has very good timing resolution (50 ps r.m.s. for one photoelectron) and shows very good temperature stability. It is also insensitive to magnetic fields. These characteristics mean that the SiPM can compete with other known photodetectors (e.g., PMT, APD, HPD, VLPC) and may prove useful for many applications, from very low light intensity detection in particle physics and astrophysics, through fast luminescence and fluorescence studies with low photon numbers in chemistry, biology and material science, to fast communication links. The main advantage of the SSPM is its small size (1×1 mm) and its low operating voltage of ˜60 V. These characteristics render SSPM ideal for use in intraoperative and intra-luminal radiation detection probes and cameras such as shown in FIG. 24.
One currently proposed medical applications for SiPM is in a small field of view PET scanner that can work in high magnetic fields of an MRI scanner (Rubashov, I. B., U.S. Pat. No. 6,946,841).